1. Field of the Invention
The present invention relates to an external additive for use in a toner for electrophotography, a toner for electrophotography, a developer for electrophotography, an image-forming process, and an image-forming apparatus.
2. Description of the Related Art
A typical image-forming process according to electrophotography or electrostatic printing comprises a developing step, a transferring step, and an image-fixing step. The developing step includes uniformly charging a photoconductive insulative layer, irradiating radiation to the insulative layer, dissipating charges on exposed portions to thereby form a latent electrostatic image, and supplying toners formed of charged fine particles to the latent electrostatic image to thereby visualize the image. The transferring step includes transferring the visualized image onto an image transfer member such as a transfer paper. The image-fixing step includes fixing the image by heating and/or pressurizing generally using a heat roller. Developers for developing latent electrostatic images formed on a latent electrostatic image bearing member, used in the developing step, include double-component developers each comprising a carrier and a toner, and single-component developers (magnetic toners and nonmagnetic toners) that do not require a carrier. Popular full-color image forming apparatus are of a system in which toner images of individual colors formed on a photoconductor are sequentially transferred and disposed to an intermediate transfer, and all of the transferred images are transferred again to a transfer paper by one operation.
A toner for use in such electrophotography and electrostatic printing mainly comprise a binder resin and a coloring agent and may further comprise a charge control agent, an offset-preventing agent, and other additives according to necessity. These components of the toner must have various capabilities and properties in the individual steps. For example, to allow toners to be disposed to a latent electrostatic image in the developing step, the toner particles and the binder resins must maintain an appropriate charge suitable for use in copying machines or printers regardless of temperature, humidity, and other surrounding conditions. In the image-fixing step using a heat roller, the toners must have satisfactory anti-offset performance so as not to adhere to a heat roller having temperature of about 100° C. to 230° C., and must have high image-fixing properties to paper. In addition, the toners must have satisfactory blocking resistance so as not to induce blocking during storage in a copying machine.
Various attempts have been made in the electrophotographic technologies to improve image quality further. Among them, downsized and spherical toners are believed to be very effective to improve image quality. However, such downsized toners may invite decreased image transfer properties and thereby yield images of poor quality. In contrast, spherical toners have improved image transfer properties (Japanese Patent Application Laid-Open (JP-A) No. 09-258474). Under these circumstances, demands have been made on image formation procedures at a higher speed in color copying machines and color printers. To form images at a higher speed, a “tandem system” is effective (refer to, for example, JP-A No. 05-341617). In the tandem system, images formed by an image-forming unit are sequentially transferred and superimposed onto a single transfer paper transported by a transfer belt (transfer) to thereby form a full-color composite image on the transfer paper. Such a color image forming apparatus according to the tandem system can use a wide variety of transfer papers and can yield full-color images with high quality at a high speed. In particular, the apparatus can yield full-color images at a higher speed than conventional color image forming apparatus according to the other systems. Another attempt has been made to form images at a high speed while yielding high image quality using a spherical toner. If an apparatus according to this system is operated at a higher speed, the toner must pass through the development unit in a shorter time. A toner for use herein must therefore be transferred at a higher pressure so as to yield similar image transfer capabilities to conventional equivalents. However, transfer at such a higher pressure invites aggregation of the toner particles. As a result, the toner cannot be transferred satisfactorily and may invite hollow defects in the formed images. To solve these problems and to achieve higher image quality, the circularity, particle diameter, specific gravity, BET specific surface area of the toner are specified, and the deposition stress of the toners when compressed at 1 kg/cm2 is specified to 6 g/cm2 or less (e.g., JP-A No. 2000-3063, or the like).
However, when the deposition stress of the toners under compression at 1 kg/cm2 is used as a specific parameter, the compression pressure is too low to thereby invite insufficient image transfer properties and hollow defects of characters when the transfer pressure increases by the use of, for example, overhead projector (OHP) films or sheets, cardboard or surface coated paper, or the like. In addition, such a low deposition stress of the toners may invite transfer dust, or the like. To exhaust the toners more sufficiently, an attempt has been made to specify the deposition force per particle of the toners at 3.0 dyne/contact-point or less (JP-A No. 2000-352840). However, this technique does not specify the deposition force of the toner under compression and thereby does not effectively improve image transfer properties and image quality to avoid, for example, hollow defects of characters, although the toner is exhausted more sufficiently.
To develop images more satisfactorily with higher stability with time, the aggregation degree of toners under compression is specified (Japanese Patent (JP-B) No. 3002063). However, specifying the aggregation degree of the toners under compression may still invite hollow defects of character images, may invite insufficient image quality and does not sufficiently improve the image transfer properties and transfer ratio. Another attempt to solve the problems on hollow defects of character images has been made by specifying the product of the aggregation degree and the loose apparent density of a toner at 7 or less (JP-A No. 2000-267422). However, this technique does not take physical behaviors of the toners under compression into consideration and does not yield sufficient effect in an intermediate image transfer system or a development system with strong stirring in which the toners undergo higher stress. Yet another attempt has been made to specify the ratio of the loosen apparent density to the hardened apparent density of a toner at 0.5 to 1.0 and to specify the aggregation degree at 25% or less (JP-A No. 2000-352840). The hardened apparent density used in this technique is the bulk density of the toners after tapping 50 times. The hardened apparent density is substantially a physical property reflecting the fluidity of the toners and does not reflect factors to increase the bulk density of the toner when mechanical stress is applied thereto. Accordingly, the technique does not yield sufficient effect in an intermediate image transfer system or a development system with strong stirring in which the toners undergo higher stress.
To improve fluidity and electrostatic properties of toners, “external additives” such as metal oxide particles and other inorganic powder (particles) are added to the toners. To modify hydrophobicity, electrostatic properties, and other properties of the surface of the inorganic powders, the surface of the inorganic particles is treated with a specific silane coupling agent, a titanate coupling agent, silicone oil, an organic acid, or the like, or is coated with a specific resin. Examples of the inorganic powder (particles) are powders of silicon dioxide (silica), titanium dioxide (titania), aluminum oxide, zinc oxide, magnesium oxide, cerium oxide, iron oxide, copper oxide, tin oxide, and the like.
Among them, hydrophobic silica fine particles or titanium oxide fine particles are often used. Such hydrophobic silica fine particles or titanium oxide fine particles are prepared by allowing fine particles of silica or titanium oxide to react with an organosilicon compound such as dimethyldichlorosilane, hexamethyldisilazane, silicone oil, or the like, to substitute silanol groups on the surface of fine particles with organic groups.
Among such hydrophobing agents, silicone oil has sufficient hydrophobicity and enables a toner which contains the silicone oil to exhibit satisfactory image transfer properties due to its low surface energy. For example, Japanese Patent Application Publication (JP-B) No. 07-3600 and Japanese Patent (JP-B) No. 2568244 disclose the degree of hydrophobicity of silica treated with silicone oil. JP-A No. 07-271087 and JP-A No. 08-29598 disclose the amount of the silicone oil or the carbon content in the additive. The specified silicone oil content and degree of hydrophobicity disclosed in these publications are sufficient to make the treated inorganic fine particles hydrophobic and to ensure stable electrostatic properties of the developer at high humidity. However, no positive attempt has been made to utilize such specific low surface energy of the silicone oil to thereby reduce deposition of a developer to members to be in contact with the developer. Such members include a contact charging device, a developer-bearing member (development sleeve), a doctor blade, a carrier, a latent electrostatic image-bearing member (photoconductor), an Intermediate transfer, and the like. In particular, strong deposition of the developer to the photoconductor invites toner deposition on the background of images, and dropout or hollow defects after transfer (portions where the developer is not transferred) in edges or centers of characters, lines, and dots in images. In addition, when an image transfer member has considerable depressions and protrusions, images cannot satisfactorily be transferred to the depressions, thus inviting hollow defects. Simple control of the amount of the silicone oil or the degree of hydrophobicity cannot solve these problems. JP-A No. 11-212299 discloses inorganic fine particles containing a specific amount of silicone oil as a liquid component. However, the use of the silicone oil in the specified amount does not satisfy the above requirements.
The toner for electrophotography must be charged uniformly and stably. If not, the toners invite toner deposition on the background of images or non-uniform image density to thereby deteriorate image quality. A development unit has been downsized with a decreasing size of an image-forming apparatus. Sharp toner charge raise thereby increases in its importance to obtain high image quality in such a downsized development unit. To satisfy these requirements, various proposals have been made. For example, to improve electrostatic properties of toners for electrophotography by additives, JP-A No. 03-294864 discloses a non-magnetic single-component developer comprising inorganic powder treated with silicone oil; JP-A No. 04-204665 discloses a magnetic single-component developer in which an additive covers 3% to 30% of a toner; and JP-A No. 04-335357 discloses an electrostatic developer comprising a toner and an external additive, in which toner has fine particles with a BET specific surface area of 5 m2/g to 100 m2/g fixed on its surface, and the external additive comprises particles having a specific surface area of 1.2 times or more that of the fine particles fixed on the toner. JP-A No. 07-43930 discloses a developer using a non-magnetic single-component toner including hydrophobic silica fine particles and specific hydrophobic titanium oxide; and JP-A No. 08-202071 discloses a developer which contains a toner additive comprising organic-inorganic composite particles having an organic polymer skeleton and a polysiloxane skeleton.
However, even these techniques cannot yield sufficiently uniform charges and satisfactorily sharp toner charging rise and are not sufficient in stability in surroundings of toner charge, particularly in stability of toner charge at high humidity. Most of these techniques employ an additive having improved hydrophobicity as a result of a surface treatment of regular oxide particles. The use of such an additive, however, invites deterioration of the toner due to a varying composition of the additive with time during operation, although the toner exhibits a desired stable charging at early stages. The composite particles prepared by a liquid phase process as disclosed in JP-A No. 08-202071 may not have sufficient hydrophobicity and may exhibit varying hydrophobicity with time due to a mediating substance (medium) remained inside the particles.
To avoid color misregistration, to stabilize image densities and image transfer properties for a long time and to avoid toner contamination, large-diameter inorganic fine particles having an average particle diameter of 50 nm to 120 nm are added to a toner (Japanese Patent No. 3148950). However, this technique does not have the effects of improving sharp charging rise after printing a number of sheets and of avoiding toner deposition on the background of images at high temperatures and high humidity or at low temperatures and low humidity.
A toner additive for electrophotography has been developed. This toner additive comprises oxide fine particles obtained by oxidizing solid solution fine particles containing two or more elements, has a minimum difference in first ionization potential between elements contained in the solid solution fine particles of from 1.20 eV to 4.20 eV and a maximum first ionization potential among the elements contained in the solid solution fine particles of 9.00 eV or less. However, this technique does not sufficiently consider the particle diameter and shape of the inorganic fine particles and does not yield satisfactory fluidity, image transfer properties, and the toner according to this technique is not sufficiently stirred upon development, merely by specifying the ionization potential alone.
Binder resins for use in toners must have transparency, insulating properties, water resistance, fluidity (fluidability) as a powder, mechanical strength, glossiness, thermoplasticity, grindability, and or the like. To satisfy these requirements, polystyrenes, styrene-acrylic copolymers, polyester resins, and epoxy resins are generally used as the binder resins. Among them, styrenic resins are widely used for their satisfactory grindability, water resistance, and fluidity. However, when a copy obtained by using a toner which contains a styrenic resin is stored in a paper holder made of a vinyl chloride resin sheet, an image-bearing surface of the copy is left in intimate contact with the sheet. A plasticizer contained in the vinyl chloride resin sheet then migrates into and plasticizes the fixed toner image to thereby allow the toner image to adhere to the sheet. When the copy is taken out from the sheet, a part or whole of the toner image is peeled off from the copy and causes toner deposition on the sheet. This problem also occurs in a toner which contains a polyester resin.
To avoid adhesion of the toner to such a vinyl chloride resin sheet, JP-A No. 60-263951 and JP-A No. 61-24025 disclose blending of an epoxy resin with a styrenic resin or polyester resin, since such an epoxy resin is not plasticized by a plasticizer for vinyl chloride resins.
However, when the blended resin is used in a color toner, the resulting toner cannot satisfy all the requirements in anti-offset performance, resistance to curling of fixed images, glossiness, colorability, transparency, and color reproducibility. For example, if a color toner image has insufficient glossiness, it is seen unsubstantial. Conventional epoxy resins and acetylated modified epoxy resins disclosed in JP-A No. 61-235852 do not satisfy all these requirements.
A possible solution to these problems is a single use of an epoxy resin. However, such epoxy resins have high reactivity with amines. The epoxy resins are generally used as curable resins having satisfactory mechanical strength and chemical resistance. These properties are derived from their crosslinked structure formed as a result of a reaction between epoxy groups and a curing agent. Such curing agents are roughly classified into amine curing agents and organic acid anhydride curing agents. Naturally, an epoxy resin for use in a toner for electrostatic development is used as a thermoplastic resin. However, some of dyes, pigments, and charge control agents to be kneaded with the resin to yield a toner are amine-containing agents and invite a crosslinking reaction during kneading. The resulting crosslinked article cannot be used as a toner. In addition, the chemical activity of the epoxy groups may potentially induce biochemical toxicity such as skin irritation that must be avoided.
In addition, such epoxy groups are hydrophilic and the resulting toner markedly absorbs water at high temperatures and high humidity, thus causing a decreased charge, toner deposition on the background of images, and insufficient cleaning. The epoxy resins also have insufficient electrostatic stability.
Regular toners each comprise a binder resin, a coloring agent, a charge control agent, and the like. Such coloring agents include various dyes and pigments, and some of them have charge control properties and thereby play a role both as a coloring agent and a charge control agent. Such toners having the above composition are often prepared using an epoxy resin as the binder resin. In these toners, the dye or pigment, the charge control agent, and other additives must be sufficiently dispersed. More specifically, the dye or pigment and the charge control agent are generally kneaded with the binder resin in a heat roll mill and must be uniformly dispersed in the binder resin. However, it is difficult to disperse these components uniformly. If the dye or pigment as the coloring agent is not sufficiently dispersed, the toner may exhibit insufficient color development and decreased colorability (degree of coloring). If the charge control agent is not sufficiently dispersed, charges distribute non-uniformly, thus inviting various defects or failures such as charging failure, toner deposition on the background of images, scattering of toners, insufficient image density, lack of ID, fuzzing, and insufficient cleaning. JP-A No. 61-219051 discloses a toner using an ester-modified epoxy resin, which is modified with ε-caprolactone, as the binder resin. The epoxy resin used herein is modified in a high magnitude of 15% by weight to 90% by weight, and the resulting toner has an excessively low softening point and excessively high glossiness, although it has improved resistance to vinyl chloride resins and fluidity.
JP-A No. 52-86334 discloses an epoxy resin having positive charges prepared by allowing terminal epoxy groups of a prepared epoxy resin to react with an aliphatic primary or secondary amine. However, the epoxy group may crosslink with such an amine as described above, and the resulting resin may not be used as a toner. JP-A No. 52-156632 discloses that one or both of terminal epoxy groups of an epoxy resin are allowed to react with alcohol, phenol, a Grignard reagent, an organic acid sodium acetylide, or an alkyl chloride. However, a residual epoxy group, if any, may invite problems such as reactivity with amines, toxicity, and hydrophilicity. In addition, some of the aforementioned reaction products are hydrophilic, affect electrostatic properties, or affect grindability in the preparation of toners, and are not always effective to satisfy all of the requirements.
JP-A No. 01-267560 discloses a modified epoxy resin prepared by allowing both of terminal epoxy groups of an epoxy resin to react with a monovalent compound having an active hydrogen and esterifying the reaction product with a monocarboxylic acid, an ester derivative or a lactone derivative thereof. The resulting epoxy resin does not exhibit sufficiently improved resistance to curling in image-fixing although problems in the reactivity, toxicity and hydrophilicity of the epoxy resin are solved.
Xylene and other solvents are often used in preparation of epoxy resins or polyol resins as disclosed in JP-A No. 11-189646, for example. However, these solvents and unreacted residual monomers such as bisphenol A remain in a significantly large amount in the produced resins and consequently in toners using the resins.
Toners for electrophotography are generally produced by a process comprising the steps of mixing all materials in one step, heating, melting, and dispersing the resulting mixture to yield a homogenous composition, cooling, pulverizing, and classifying the composition to thereby yield a toner having a volume-average particle diameter of 6 μm to 10 μm. A typical disclosure of the method can be found in JP-A No. 01-304467. In particular, color toners for electrophotography for use in the formation of color images generally comprise a chromatic dye or pigment dispersed in a binder resin and require more strict performances than those for use in the formation of black images. Specifically, the color toners must have satisfactory and appropriate color development (colorability), and optical transparency when used in over head projector (OHP) transparencies, in addition to mechanical and electrical stability to external factors such as impact and humidity. Certain toners using a dye as a coloring agent are disclosed, for example, in JP-A No. 57-130043, JP-A No. 57-130044, or the like.
However, these toners using a dye as a coloring agent have insufficient lightfastness and undergo discoloring or fading when they are left under direct radiation, although they can yield sharp color images with high transparency and good color development.
Image-forming apparatus according to an intermediate image transfer system are typically known, in which visible toner images of different colors sequentially disposed on an image bearing member, are then sequentially primarily transferred onto an Intermediate transfer moved on an endless member and are temporarily held thereon, and the transferred images (toner images) are then secondly transferred onto a transfer sheet by one operation. These image forming apparatus according to the intermediate image transfer system are advantageous in that they can be downsized and a wide variety of transfer materials (transfer paper) can be used for transferring visible images and are therefore frequently used as color image forming apparatus. Such image-forming apparatus may invite worm-eaten defects in images (hollow defects of characters) on an image transferring medium such as transfer paper serving as a final image forming medium. In these hollow defects, no toner is transferred to certain locations due to local omission of transferred toner images constituting the color developed images in primary transfer and secondary transfer. In solid images, the worm-eaten images form transfer omission with certain areas. In line images, they invite transfer omission to thereby form broken line images.
Such irregular images often occur in the formation of full four-color images. This is because a toner layer herein has a larger thickness and strong non-Coulomb mechanical adhesive force (van der Waals force and other force except electrostatic force) produces due to contact pressure between the surface of an image bearing member and the toners, or between the surface of the intermediate transfer and the toners, since the primary transfer procedure is repeated at maximum four times. In addition, filming phenomenon occurs in the repeated image forming process. In the filming phenomena, a film of the toner is deposited on the surface of the Intermediate transfer and thereby adhesive force between the Intermediate transfer and the toner increases.
To avoid such images with hollow defects, commercially available machines have already employed techniques of applying a lubricant to the surface of an image bearing member and Intermediate transfer to thereby reduce the adhesive force between the toner and these members, or of adding an external additive to a toner to thereby reduce the adhesive force of the toner itself. However, these techniques do not consider the adhesive force among toner particles, tensile strength at break, and other parameters upon increased transfer contact pressure during full four-color image formation or transfer at a high speed. In particular, they cannot yield stable image quality when the toner is transferred to cardboard, surface-coated paper or OHP transparencies.
JP-A No. 08-211755 discloses a technique for improving image transfer properties and for preventing abnormal images with hollow defects by controlling relative balance between the adhesive force of an image bearing member to a toner and that of an Intermediate transfer to the toner. However, the adhesive force of the toner used herein is determined by a centrifugal method using a powdery toner, and the resulting toner has different physical properties from the one under increased transfer contact pressure.
Toners after their production are exposed to severe conditions such as high temperatures and high humidity or low temperature and low humidity during storage and transportation. The toners must therefore have high storage stability with no or little deterioration in electrostatic properties, fluidity, image transfer properties, and image-fixing properties without aggregation of toners even after storage under those conditions. However, no effective solution to these requirements has been found.